In a typical cellular radio communication system (wireless communication system), an area is divided geographically into a number of cell sites, each defined by a radio frequency (RF) radiation pattern from a respective antenna or antenna system. The antennas in the cells are in turn coupled to one or another form of controller, which is then coupled to a telecommunications switch or gateway, such as a mobile switching center (MSC) and/or a packet data serving node (PDSN) for instance. These (and possibly other) elements function collectively to form a Radio Access Network (RAN) of the wireless communication system. The switch or gateway may then be coupled with a transport network, such as the PSTN or a packet-switched network (e.g., the Internet).
Depending on the specific underlying technologies and architecture of a given wireless communication system, the RAN elements may take different forms. In a code division multiple access (CDMA) system configured to operate according IS-2000 and IS-856 standards, for example, the antenna system is referred to as a base transceiver system (BTS), and is usually under the control of a base station controller (BSC). In a universal mobile telecommunications system (UMTS) configured to operate according to ITU IMT-2000 standards, the antenna system is usually referred to as a NodeB, and is usually under the control of a radio network controller (RNC). Other architectures and operational configurations of a RAN are possible as well.
A subscriber (or user) in a service provider's wireless communication system accesses the system for communication services via a communication device, such as a cellular telephone, “smart” phone, pager, or appropriately equipped portable computer, for instance. In a CDMA system the communication device is referred to as an access terminal (also referred to herein by “AT”); in a UMTS system the communication device is referred to as user equipment (also referred to herein by “UE”). When an AT or UE is positioned in a cell, it communicates via an RF air interface with the BTS or NodeB antenna of the cell. Consequently, a communication path or “channel” is established between the AT or UE and the transport network, via the air interface, the BTS or NodeB, the BSC or RNC, and the switch or gateway.
As the demand for wireless communications has grown, the volume of call traffic in most cell sites has correspondingly increased. To help manage the call traffic, most cells in a wireless network are usually further divided geographically into a number of sectors, each defined respectively by radiation patterns from directional antenna components of the respective BTS or NodeB, or by respective antennas. These sectors can be referred to as “physical sectors,” since they are physical areas of a cell site. Therefore, at any given instant, an AT or UE in a wireless network will typically be positioned in a given physical sector and will be able to communicate with the transport network via the BTS or NodeB serving that physical sector.
The functional combination of a BTS of a cell or sector with a BSC, or of a NodeB and an RNC, is commonly referred to as a “base station.” The actual physical of a configuration of a base station can range from an integrated BTS-BSC or NodeB-RNC unit to a distributed deployment of multiple BTSs under a single BSC, or multiple NodeBs under a single RNC. A base station may be typically deployed to provide coverage over a geographical area on a scale of a few to several square miles and for tens to hundreds to several thousands (or more) of subscribers at any one time.
As a subscriber at an AT or UE moves between wireless coverage areas of a wireless communication system, such as between cells or sectors, or when network conditions change or for other reasons, the AT or UE may “hand off” from operating in one coverage area to operating in another coverage area. In a usual case, this handoff process is triggered by the AT or UE monitoring the signal strength of various nearby available coverage areas, and the BSC or RNC (or other controlling network entity) determining when one or more threshold criteria are met. For instance, an AT may continuously monitor signal strength from various available sectors and notify a BSC when a given sector has a signal strength that is sufficiently higher than the sector in which the AT is currently operating. The BSC may then direct the AT to hand off to that other sector. By convention, an AT or UE is said to handoff from a “source” cell or sector (or base station) to a “target” cell or sector (or base station).
In some wireless communication systems or markets, a wireless service provider may implement more than one type of air interface protocol. For example, a carrier may support one or another version of CDMA, such as EIA/TIA/IS-2000 Rel. 0, A, and CDMA 2000 Spread Spectrum Systems Revision E (collectively referred to generally herein as “IS-2000”) for both circuit-cellular voice and data traffic, as well as a more exclusively packet-data-oriented protocol such as EIA/TIA/IS-856 Rel. 0, A, or other version thereof (hereafter “IS-856”). A carrier could also implement an orthogonal frequency division multiple access (OFDMA) based system according to protocols specified by third generation partnership project (3GPP) Long Term Evolution (“LTE”) Advanced, for example. Access terminals or UEs may be capable of communication under any or all such protocols, and may further be capable of handing off between them, in addition to being able to hand off between various configurations of coverage areas.